There are many arrangements for adding an ancillary code to a signal in such a way that the added code is not noticed. For example, it is well known in television broadcasting that ancillary codes can be hidden in non-viewable portions of video by inserting the codes into either the video's vertical blanking interval or the video's horizontal retrace interval. An exemplary system that hides codes in non-viewable portions of video is referred to as “AMOL” and is taught in U.S. Pat. No. 4,025,851. This system is used by the assignee of the present application in order to monitor broadcasts of television programming as well as the times of such broadcasts.
Other known video encoding systems have sought to bury ancillary codes in a portion of a television signal's transmission bandwidth that otherwise carries little signal energy. Dougherty in U.S. Pat. No. 5,629,739, which is assigned to the assignee of the present application, discloses an example of such a system.
It is also known to add ancillary codes to audio signals for the purpose of identifying the signals and, perhaps, for tracing their courses through signal distribution chains. Audio encoding has the obvious advantage of being applicable not only to television, but also to radio broadcasts and to prerecorded music. Moreover, the speaker of a receiver reproduces, in the audio signal output, the ancillary codes that are added to audio signals. Accordingly, audio encoding offers the possibility of non-intrusive interception (i.e., interception of the codes without intrusion into the interior of the receiver) and of decoding the codes with equipment that has microphones as inputs. Moreover, audio encoding permits the measurement of broadcast audiences by the use of portable metering equipment carried by panelists.
In the field of audio signal encoding for broadcast audience measurement purposes, Crosby, in U.S. Pat. No. 3,845,391, teaches an audio encoding approach in which the code is inserted in a narrow frequency “notch” from which the original audio signal is deleted. The notch is made at a fixed predetermined frequency (e.g., 40 Hz). This approach leads to codes that are audible when the original audio signal containing the code is of low intensity.
A series of improvements followed the Crosby patent. Thus, Howard, in U.S. Pat. No. 4,703,476, teaches the use of two separate notch frequencies for the mark and the space portions of a code signal. Kramer, in U.S. Pat. Nos. 4,931,871 and in U.S. Pat. No. 4,945,412 teaches, inter alia, using a code signal having an amplitude that tracks the amplitude of the audio signal to which the code is added.
Broadcast audience measurement systems in which panelists are expected to carry microphone-equipped audio monitoring devices that can pick up and store inaudible codes broadcast in an audio signal are also known. For example, Aijalla et al., in WO 94/11989 and in U.S. Pat. No. 5,579,124, describe an arrangement in which spread spectrum techniques are used to add a code to an audio signal. The code is either not perceptible, or can be heard only as low level “static” noise.
Also, Jensen et al., in U.S. Pat. No. 5,450,490, teach an arrangement for adding a code at a fixed set of frequencies and using one of two masking signals. The choice of masking signal is made on the basis of a frequency analysis of the audio signal to which the code is to be added. Jensen et al. do not teach arrangements for selecting a maximum acceptable code energy to be used in each of a predetermined set of frequency intervals, nor do Jensen et al. teach energy exchange coding which transfers energy between spectral components and which thereby holds the total acoustic energy constant.
Preuss et al., in U.S. Pat. No. 5,319,735, teach a multi-band audio encoding arrangement in which a spread spectrum code is inserted in recorded music at a fixed ratio to the input signal intensity (code-to-music ratio) that is preferably 19 dB. Lee et al., in U.S. Pat. No. 5,687,191, teach an audio coding arrangement suitable for use with digitized audio signals. The code intensity is made to match the input signal by calculating a signal-to-mask ratio in each of several frequency bands and by then inserting the code at an intensity that is a predetermined ratio of the audio input in that band. Lee et al. has also described a method of embedding digital information in a digital waveform in U.S. Pat. No. 5,824,360.
Jensen et al., in U.S. Pat. No. 5,764,763, teach a method in which code signals consisting of sinusoidal waves at ten pre-selected frequencies in a high resolution spectrum are added to the original audio in order to represent either a binary bit (0 or 1) and the start and end of an embedded message. Forty unique frequencies are required for encoding these four symbols. Their values range from 1046.9 Hz to 2851.6 Hz in a typical practical embodiment. The frequency separation between adjacent lines in the spectrum is 4 Hz and the minimum separation between frequencies selected to constitute the set of 40 frequencies is 8 Hz. The amplitude of the injected code signal is controlled by a masking analysis. In the decoding process, the injected code signal is distinguished by the fact that its level will be significantly above a noise level computed for a band of frequencies.
It will be recognized that, because ancillary codes are preferably inserted at low intensities in order to prevent the codes from distracting a listener of program audio, such codes may be vulnerable to various signal processing operations as well as to interference from extraneous electromagnetic sources. For example, although Lee et al. discuss digitized audio signals, many of the earlier known approaches to encoding a broadcast audio signal are not compatible with current and proposed digital audio standards, particularly those employing signal compression methods that may reduce the signal's dynamic range (and thereby delete a low level code) or that otherwise may damage an ancillary code. In this regard, it is particularly important for an ancillary code to survive compression and subsequent de-compression by the AC-3 algorithm or by one of the algorithms recommended in the ISO/IEC 11172 MPEG standard, which is expected to be widely used in future digital television broadcasting systems.
U.S. patent application Ser. No. 09/116,397 filed Jul. 16, 1998 and U.S. patent application Ser. No. 09/428,425 filed Oct. 27, 1999 disclose a system and method for inserting a code into an audio signal so that the code is likely to survive compression and decompression as required by current and proposed digital audio standards. Spectral modulation of the amplitude or phase of the signal at selected code frequencies is used to insert the code into the audio signal. These selected code frequencies, which could comprise multiple frequency sets within a given audio block, may be varied from audio block to audio block, and the spectral modulation may be implemented as amplitude modulation, modulation by frequency swapping, phase modulation, and/or odd/even index modulation. Moreover, an approach is taught to measuring audio quality of each block and of suspending encoding in cases where the code might be audible to a listener.
In experimental systems of the sort taught in the '397 application and in the '425 application, the audio sampling process during encoding imposes a delay in excess of twenty milliseconds in the audio portion of a television program. Left uncorrected, this delay results in a perceptible loss of synchronization between the audio and video portions of a viewed program. Hence, practical systems of this sort have required the use of a compensating video delay circuit. However, it is preferable to do without such a circuit.
Moreover, in systems of the sort taught in the '397 application and in the '425 application, codes are added by manipulating pairs of frequencies that are spaced apart by about 100 Hz. These systems are thus vulnerable to interference, such as reverberation or multi-path distortion, that affect one of the encoded frequencies substantially more than the other.
The present invention is arranged to solve one or more of the above noted problems.